Apparatus and method for providing a selectively absorbing structure

ABSTRACT

An apparatus is described that selectively absorbs electromagnetic radiation. The apparatus includes a conducting surface, a dielectric layer formed on the conducting surface, and a plurality of conducting particles distributed on the dielectric layer. The dielectric layer can be formed from a material and a thickness selected to yield a specific absorption spectrum. Alternatively, the thickness or dielectric value of the material can change in response to an external stimulus, thereby changing the absorption spectrum.

If an Application Data Sheet (ADS) has been filed on the filing date ofthis application, it is incorporated by reference herein. Anyapplications claimed on the ADS for priority under 35 U.S.C. §§119, 120,121, or 365(c), and any and all parent, grandparent, great-grandparent,etc. applications of such applications, are also incorporated byreference, including any priority claims made in those applications andany material incorporated by reference, to the extent such subjectmatter is not inconsistent herewith.

STATEMENT OF GOVERNMENT SUPPORT

Research concerning the subject matter disclosed herein was supported,in part, by funds from the United States Government under Air ForceOffice of Scientific Research (AFOSR) Grant Number FA9550-09-1-0562,entitled ADVANCED METACRYSTAL MEDIA FOR AEROSPACE APPLICATIONS. The U.S.Government has certain rights in the subject matter disclosed herein.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the earliest availableeffective filing date(s) from the following listed application(s) (the“Priority Applications”), if any, listed below (e.g., claims earliestavailable priority dates for other than provisional patent applicationsor claims benefits under 35 USC §119(e) for provisional patentapplications, for any and all parent, grandparent, great-grandparent,etc. applications of the Priority Application(s)). In addition, thepresent application is related to the “Related Applications,” if any,listed below.

Priority Applications

The present application is a 35 USC 371 application of PCT InternationalPatent Application No. PCT/US13/36847, filed Apr. 16, 2013 and entitledAPPARATUS AND METHOD FOR PROVIDING A SELECTIVELY ABSORBING STRUCTURE,which claims benefit of priority of U.S. Provisional Patent ApplicationNo. 61/624,571, entitled CONTROLLED REFLECTANCE SURFACES WITH COLLOIDALPLASMONIC NANOANTENNAS AND METHODS OF USE, naming DAVID R. SMITH,ANTOINE MOREAU, CRISTIAN CIRACI, AND JACK J. MOCK as inventors, filed 16APR. 2012; the disclosures of which are incorporated herein by referencein their entireties, and which was filed within the twelve monthspreceding the filing date of the present application or is anapplication of which a currently co-pending priority application isentitled to the benefit of the filing date.

Related Applications

None.

If the listings of applications provided above are inconsistent with thelistings provided via an ADS, it is the intent of the Applicant to claimpriority to each application that appears in the Priority Applicationssection of the ADS and to each application that appears in the PriorityApplications section of this application.

All subject matter of the Priority Applications and the RelatedApplications and of any and all parent, grandparent, great-grandparent,etc. applications of the Priority Applications and the RelatedApplications, including any priority claims, is incorporated herein byreference to the extent such subject matter is not inconsistentherewith.

TECHNICAL FIELD

The present subject matter relates to absorption of electromagneticradiation. More particularly, the present subject matter relates to aselectively absorbing structure.

BACKGROUND

Efficient and tunable absorption of electromagnetic radiation is usefulfor a variety of applications, such as designing controlled-emissivitysurfaces for thermo-photovoltaic devices, tailoring an infrared spectrumfor controlled thermal dissipation, and producing detector elements forimaging.

SUMMARY

An apparatus is provided for selectively absorbing electromagneticradiation. In one aspect, the apparatus includes a conducting surface, adielectric layer formed on the conducting surface, and a plurality ofconducting particles distributed on the dielectric layer. The conductingparticles may be cube-shaped and may be distributed randomly bycolloidal absorbance on the dielectric layer. The dielectric layer mayinclude material whose thickness or dielectric value changes in responseto an external stimulus, such as an applied electric field, appliedelectromagnetic radiation, presence of a chemical substance, or presenceof a molecular analyte. The dielectric layer may include nonlinear mediaor gain media. In another aspect, a method of forming such an apparatusmay include selecting a design electromagnetic wavelength for selectiveabsorption. A conducting surface is formed, and a dielectric layer isformed thereon. The dielectric layer has a thickness corresponding to aspacing parameter associated with the design wavelength. Multipleconducting particles, such as metal cubes or rods, are distributed onthe dielectric layer.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofvarious embodiments, is better understood when read in conjunction withthe appended drawings. For the purposes of illustration, there is shownin the drawings exemplary embodiments; however, the presently disclosedsubject matter is not limited to the specific methods andinstrumentalities disclosed. In the drawings:

FIG. 1 is a schematic depiction of a selectively absorbing structure;

FIG. 2 is a schematic showing example dimensions of a selectivelyabsorbing structure;

FIG. 3 is a graphical depiction of a spectral response of a selectivelyabsorbing structure; and

FIG. 4 is a schematic depiction of an electronic and optical systemincluding a selectively absorbing structure.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof In the drawings, similarsymbols typically identify similar components, unless context dictatesotherwise. The illustrative embodiments described in the detaileddescription, drawings, and claims are not meant to be limiting. Otherembodiments may be utilized, and other changes may be made, withoutdeparting from the spirit or scope of the subject matter presented here.

FIG. 1 depicts a selectively absorbing structure 100 that selectivelyabsorbs certain wavelengths of electromagnetic radiation 101, such aslight. The structure 100 is formed from a substrate 102, a conductingsurface 104, a dielectric spacer layer 106, and a plurality ofconducting particles 108. In one embodiment, the conducting surface 104is a metal film, such as gold, that is deposited on the substrate 102.The dielectric layer 106 may be fabricated from any of a wide variety ofsuitable materials, such as polyelectrolyte materials, and can have anindex of refraction greater than 1.5, although indices of refractionless than 1.5 are also acceptable. In certain embodiments, thedielectric layer 106 may be formed by optically nonlinear media.Examples of such media include second-harmonic generation materials,such as barium borate, lithium iodate, potassium niobate, galliumselenide, along with other well-known nonlinear optic materials,including organic nonlinear optical materials. In other embodiments, thedielectric layer 106 may be a gain medium or active lasing medium.Examples of such media include gallium arsendide, gallium nitride,crystals (e.g., sapphire or yttrium orthovanadate) or glasses (e.g.,silicate or phosphate glasses) doped with rare-earth ions or transitionmetal ions, along with other well-known gain media, including liquid andgas lasing media.

The conducting particles 108 may be cube-shaped (as shown in FIG. 1),rod shaped, or otherwise preferably shaped to provide a planar surfacethat is disposed parallel to the conducting surface 104, in order tosupport a gap-plasmon guided mode between the particle and conductingsurface. Those skilled in the art will appreciate that theselective-absorption properties of structure 100 is due to plasmonresonance. Plasmon resonance is associated with certain metals atinfrared, visible, and ultraviolet wavelengths. As is understood, othernon-metallic materials support plasmon-like responses at otherwavelengths. Accordingly, the term “conductor” and “conducting” is usedherein in its general meaning to encompass any specific material thatcan support a plasmon response associated with a specified wavelength.

FIG. 2 depicts example dimensions of the constituent parts of theselectively absorbing structure 100. In one embodiment, the conductingsurface 104 can be gold film of approximate 50 nm thickness. Theconducting particle 108 can be a silver nanocube with approximate edgelength of 74 nm, and with a 3-nm thick stabilizer coating 110. Thereflectance characteristics of structure 100 are determined by theseparation distance between the silver nanocubes 108 and gold film 104,by the dielectric properties of materials used to form the dielectricspacer 106 and stabilizer 110, by the uniformity of nanocube size, andby the percentage of gold film that is covered by the silver nanocubes.

The reflectance of structure 100 is particular sensitive to cube-filmspacing, thereby providing a tunable and selectively absorbing structureby varying the thickness of the spacer layer 106. Experimentalmeasurements have been taken using the structure 100 with the generalgeometries depicted in FIG. 2, and with different thicknesses of apolyelectrolyte having index of refraction 1.54. A 6 nm spacer layer 106yielded a reflectance minimum of less than 7% for normally incident 637nm light with samples having 17.1% surface coverage (see FIG. 3).Thinner spacer layers experimentally yielded reflectance minima forincident electromagnetic radiation having a wavelength between 700 and800 nm, and greater than 800 nm. Thicker spacer layers providereflectance minima for light having wavelengths less than 600 nm.

Those skilled in the art will appreciate that a designer can choosedifferent thicknesses for spacer layer 104, and thereby can selectdifferent corresponding design wavelengths for selective absorption bystructure 100. Conversely, any changes to the conformation of the spacerlayer 104 (such as changes in dielectric value or changes in thickness)will result in a different absorption spectra produced by the structure100. Accordingly, the structure 100 can be employed as a sensor. Forexample, the dielectric layer 104 can be composed of inorganic ororganic material that changes conformation in response to an externalstimulus. The corresponding change in the selectively absorbedwavelength detects the presence of that external stimulus, such as anapplied electric field, electromagnetic radiation, or the presence of achemical substance or molecular analyte.

FIG. 4 depicts the selectively absorbing structure 100 as includedwithin an electronic and optical system 200. Those skilled in the artwill appreciate that the spectral control and response provided by anabsorbing surface structure can form the basis for a number of promisingapplications, including electronic and optical systems such asthermo-photovoltaic devices, infrared spectrometric devices, thermaldetectors, imaging devices, light sources, and sensors.

In an example scenario, nanocubes in accordance with the present subjectmatter may be configured for use in biosensors and as an entirelycolloidal absorber. Alternative applications include, but are notlimited to, plasmonic patch nanoantennas for biological sensing. In anexample, the mode supported by a gap under the cubes may be reflectedback and forth by the edges of the cube, so that this gap can beconsidered as an interferometer. Any change to the velocity of the mode(by changing even slightly the spacing between the cubes and the film bywhatever mean) or any change in the reflection coefficient of the modethat may be brought by a change in the surrounding medium may produce ashift in the resonance of the cavity that can easily be detected—eitherby surface reflectance or through the resonant scattering by anindividual cube. A gap waveguide mode may be positioned between the cubeand the film that is altered by molecular binding events at the edges ofthe cube. Alternatively, the cube may sit on a bed of molecules (such asDNA, proteins, etc.) or an inorganic layer whose conformation issensitive and external stimuli triggering a change in the gap dimensionand thus a modification of the waveguide mode.

In addition to the use of a film to create controlled reflectancesurfaces, instead one can make use of engineered nanoparticles in anon-conducting, transparent host. In this case, magnetic-like scatteringnanoparticles, which may be two metallic disks or patches separated byan insulating layer, may be combined with regular metallic nanoparticlesthat provide electric scattering. By tuning the properties of thenanoparticles and their relative densities, the electric and magneticresponses can be controlled in the same manner as the film-coupledpatches. The result is a controlled reflectance surface that can becreated without the use of a metallic film, which may be advantageousfor certain applications.

It is noted that the application of metals in photonic systems can behampered by the relatively large absorption that results when resistivecurrents are excited. However, classes of applications exist in which,rather than being a hindrance, absorption may be advantageous, orperhaps even a requisite property. Such applications include the designof controlled emissivity surfaces for increasing thermophotovoltaicefficiency; tailoring of the infrared spectrum for controlled thermaldissipation; tailoring of the infrared spectrum for signature control;and detector elements for imaging.

Numerous metamaterial- and plasmonic-based “perfect absorbers” includemetallic surfaces that are patterned with micro- or nano-scalestructures that act as magnetic resonators. By tuning and optimizing themagnetic resonances, the electric currents of a metallic sheet can bebalanced with effective magnetic currents, and the composite structureno longer reflects at the targeted wavelength. Note that the term“perfect absorber” is primarily descriptive, and refers to surfaces thatcan be engineered so as to minimize reflectance over a specified band offrequencies. These surfaces may also be referred to as “idealabsorbers,” since they make sue of match electric and magnetic response.The minimum reflectance can be quite good, with up to 99.5% of lightabsorbed at a specific design wavelength and for a specific angle ofincidence. By implementing combinations of structures on a film, thespectral characteristics of a surface can be controlled with greatflexibility.

Many ideal absorber structures involve the use of lithographicpatterning, which does not feasibly scale well to large areas, as may beneeded for certain applications. Colloidally-prepared nanocubes inaccordance with embodiments of the present disclosure can be spacedclosely to a metal film, as a way of forming a perfect absorbingsurface. Colloidal preparation can be inexpensive, and offers analternative route for creating surfaces with controlled reflectance oremissivity properties. The underlying mechanism of the nanocubes may bethat of a patch antenna, so that any nanoparticle that is reasonablyflat, such as a pancake, disk, or the like, can be used to controlreflectance. Since the underlying effect does not rely on periodicity, arandom coverage of nanoparticles, brought to sufficient densities thatcan be easily calculated in accordance with the present disclosure, canserve to produce perfect absorbing surfaces.

In accordance with embodiments of the present disclosure, the geometrymay be that of a thin metallic film (e.g., made of gold) covered by aninsulating dielectric with controlled thickness, on which are depositedmetallic nanoparticles with the necessary geometry and density. Silveror gold are examples of metals that can be used for the nanoparticles.Because the underlying mechanism of the absorber effect relates to modesthat are excited within the gap between the metallic nanoparticle andfilm, a wide variety of materials can be used (e.g., platinum, titanium,aluminum, copper, and the like), with the controlled reflectanceproperties turned to nearly any wavelength region in the visible,infrared, and terahertz ranges.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed:
 1. A method comprising: selecting a design wavelength;determining a spacing parameter corresponding to the design wavelength;forming a conducting surface; forming a dielectric layer on theconducting surface corresponding to the spacing parameter; anddistributing a plurality of conducting particles on the dielectriclayer.
 2. The method of claim 1, wherein selecting the design wavelengthincludes selecting an electromagnetic wavelength for selectiveabsorption.
 3. The method of claim 2, wherein selecting theelectromagnetic wavelength includes selecting a wavelength greater than700 nm.
 4. The method of claim 2, wherein selecting the electromagneticwavelength includes selecting a wavelength less than 700 nm.
 5. Themethod of claim 1, wherein forming the conducting surface includesdepositing a metal film on a substrate.
 6. The method of claim 1,wherein forming the conducting surface includes forming a substantiallyplanar conducting surface.
 7. The method of claim 1, wherein forming thedielectric layer includes forming a dielectric layer of substantiallyuniform thickness, the thickness corresponding to the spacing parameter.8. The method of claim 7, wherein forming the dielectric layer ofsubstantially uniform thickness includes forming a dielectric layer ofthickness greater than 10 nm.
 9. The method of claim 7, wherein formingthe dielectric layer of substantially uniform thickness includes forminga dielectric layer of thickness less than 10 nm.
 10. The method of claim1, wherein distributing the conducting particles includes distributing aplurality of conducting particles that each have at least onesubstantially planar surface, and wherein the planar surface is orientedsubstantially parallel to the conducting surface.
 11. The method ofclaim 1, wherein distributing the conducting particles includesdistributing a plurality of cube-shaped particles on the dielectriclayer.
 12. The method of claim 11, wherein distributing the cube-shapedparticles includes distributing a plurality of cubed-shape particleshaving a linear dimension greater than 70 nm.
 13. The method of claim11, wherein distributing the cube-shaped particles includes distributinga plurality of cubed-shape particles having a linear dimension less than70 nm.
 14. The method of claim 1, wherein distributing the conductingparticles includes distributing a plurality of rod-shaped particles onthe dielectric layer.
 15. The method of claim 1, wherein distributingthe conducting particles includes randomly distributing a plurality ofconducting particles on the dielectric layer.
 16. The method of claim15, wherein randomly distributing the conducting particles includescolloidally adsorbing a plurality of conducting particles on thedielctric layer.
 17. The method of claim 1, wherein distributing theconducting particles includes distributing a plurality of particles tocover greater than 10% of the conducting surface.
 18. The method ofclaim 1, wherein distributing the conducting particles includesdistributing a plurality of particles to cover less than 10% of theconducting surface.
 19. The method of claim 1 wherein forming thedielectric layer includes forming a layer of optically nonlinearmaterial.
 20. The method of claim 1 wherein forming the dielectric layerincludes forming a layer including a gain medium.
 21. An apparatus forselective absorption of electromagnetic radiation, the apparatuscomprising: a conducting surface; a dielectric layer formed on theconducting surface; and a plurality of conducting particles distributedon the dielectric layer.
 22. The apparatus of claim 21, wherein theconducting surface is a substantially planar conducting surface.
 23. Theapparatus of claim 21, wherein the conducting surface is a metal film.24. The apparatus of claim 21, wherein the dielectric layer is asubstantially uniformly thick layer.
 25. The apparatus of claim 24,wherein the thickness of the dielectric layer is greater than 10 nm. 26.The apparatus of claim 24, wherein the thickness of the dielectric layeris less than 10 nm.
 27. The apparatus of claim 24, wherein the thicknessof the dielectric layer corresponds to a design wavelength ofelectromagnetic radiation for selective absorption.
 28. The apparatus ofclaim 27, wherein the design wavelength is greater than 700 nm.
 29. Theapparatus of claim 27, wherein the design wavelength is less than 700nm.
 30. The apparatus of claim 21, wherein the conducting particles arecube-shaped conducting particles.
 31. The apparatus of claim 30, whereinthe cube-shaped conducting particles have linear dimension greater than70 nm.
 32. The apparatus of claim 30, wherein the cube-shaped conductingparticles have linear dimension less than 70 nm.
 33. The apparatus ofclaim 21, wherein the conducting particles are rod-shaped conductingparticles.
 34. The apparatus of claim 21, wherein the conductingparticles are randomly distributed on the dielectric layer.
 35. Theapparatus of claim 21, wherein the conducting particles cover greaterthan 10% of the conducting surface.
 36. The apparatus of claim 21,wherein the conducting particles cover less than 10% of the conductingsurface.
 37. The apparatus of claim 21, wherein the dielectric layerincludes organic material.
 38. The apparatus of claim 21, wherein thedielectric layer includes material that changes thickness in response toan external stimulus.
 39. The apparatus of claim 38, wherein theexternal stimulus is a selected one of an applied electric field,electromagnetic radiation a chemical substance, and a molecular analyte.40. The apparatus of claim 21, wherein the dielectric layer includesmaterial that changes in dielectric value in response to an externalstimulus.
 41. The apparatus of claim 40, wherein the external stimulusis a selected one of an applied electric field, electromagneticradiation, a chemical substance, and a molecular analyte.
 42. Theapparatus of claim 21, wherein the dielectric layer includes opticallynonlinear material.
 43. The apparatus of claim 21, wherein thedielectric layer includes a gain medium.
 44. An apparatus comprising anelectronic and optical system having a selective-absorption device, theselective absorption device comprising: a conducting surface; adielectric layer formed on the conducting surface; and a plurality ofconducting particles distributed on the dielectric layer.
 45. Theapparatus of claim 44, wherein the electronic and optical systemincludes a thermo-photovoltaic device.
 46. The apparatus of claim 44,wherein the electronic and optical system includes an infraredspectrometer.
 47. The apparatus of claim 44, wherein the electronic andoptical system includes an imaging device.
 48. The apparatus of claim44, wherein the electronic and optical system includes a beam formingdevice.
 49. The apparatus of claim 44, wherein the electronic andoptical system includes a sensor.